As illustrated in FIG. 16, an existing IMS network supported by an operation and charging support system 6 is divided into three layers of an access layer 9, a session control layer 8 and an application layer 7, the access layer 9 includes a GPRS Gateway Support Node (GGSN) 91, a Serving GPRS Support Node (SGSN) 92 and an IP access network 93, and a user equipment accessing the access layer 9 may enjoy a service provided by an IMS system. The IMS system located at the session control layer 8 in the network supports an IMS Service Control (ISC) interface 75, via which the IMS system may support application servers 71 and 73 based upon the Session Initiation Protocol (SIP) providing a supplementary service and a self-owned value-added service, and support a service capability server 74 based upon the Open Service Architecture (OSA) providing a third-party service as well as an original intelligence network service platform 72. The IMS system 10 having a structure as illustrated in FIG. 17 includes physical entities of a Proxy-Call Session Control Function (P-CSCF) module 161, an Interrogating-Call Session Control Function (I-CSCF) module 163, a Service-Call Session Control Function (S-CSCF) module 162, a Media Gateway Control Function (MGCF) module 14, a Multimedia Resource Function Controller (MRFC) 13, a Multimedia Resource Function Processor (MRFP) 11, an IMS Media Gateway (IMS-MGW) 12, a Breakout Gateway Control Function (BGCF) module 15, a Home Subscriber Server (HSS) 17, etc., where the S-CSCF 162 is a control core of the IMS system, the P-CSCF 161 is an entrance for an IMS user to access the network, the P-CSCF 161 forwards a service request from the user to the S-CSCF 162 for processing by a service platform of a home network; the MGCF 14 and the IMS-MGW 12 provide interconnectivity between the IMS system 10 and a Universal Mobile Telecommunications System (UMTS) network Circuit Switched (CS) domain or a Public Switched Telephone Network (PSTN) 1; the MRFC 13 and the MRFP 11 provide media resources and relevant control functions required in a service; and the BGCF 15, which is an interface point between an IMS network 2′ and an external network, selects a connection point with the PSTN 1 of the CS domain. A legacy mobile signaling network 3 is connected with the HSS 17 via interfaces Gr, Gc, C and D; the IMS network 2′ and the MRFP 11 are connected via an interface Mb, and the MRFP 11 and the IMS-MGW 12 are connected via an interface Mb; the PSTN 1 is connected with the IMS-MGW 12 and the MGCF 14 via interfaces PSTN; the IMS-MGW 12 and the MGCF 14 are connected via an interface Mn; the BGCF 15 is connected with another BGCF 15 and the IMS network 2′ via interfaces Mk; the BGCF 15 and the MGCF 14 are connected via an interface Mj; the MGCF 14 and the S-CSCF 162 are connected via an interface Mg; the MRFP 11 and the MRFC 13 are connected via an interface Mp; the MRFC 13 and the S-CSCF 162 are connected via an interface Mr; the BGCF 15 and the S-CSCF 162 are connected with an interface Mi; the S-CSCF 162 is connected with the P-CSCF 161 and the I-CSCF 163 via interfaces Mw; the IMS network 2′ and the S-CSCF 162 are connected via an interface Mm; the S-CSCF 162 is connected with the HSS 17, the SLF 18 and an application server (AS) 5 respectively via interfaces Cx, Dx and ISC; the P-CSCF 161 and a user equipment 4 are connected via an interface Gm; and the AS 5 is connected with the HSS 17 and the SLF 18 respectively via interfaces Sh (Si) and Dh. Reference may be made to the standard 3GPP TS 23.002 for details of the foregoing interfaces.
The existing IMS system suffers from such drawbacks that the system is of such a complicated structure that excessive operations are involved in a call signaling processing flow, resulting in a prolonged flow of establishing a session. For example, to establish a call session, an initial call request (Invite) message transmitted from a calling user firstly arrives at a P-CSCF of the calling user, which in turn routes the Invite message to an S-CSCF of a calling user to trigger a service; then an I-CSCF in a network of the called user determines a home S-CSCF of the called user, and the Invite message arrives at the home S-CSCF of the called user to trigger the service and then is transmitted to a home P-CSCF of the called user and finally to the called user. If both the calling user and the called user are roaming, the call signaling flow will be even more complicated to cause more signaling interactions in the network and hence an increased delay during the session establishment procedure.